OXIDATION-REDUCTION EQUILIBRIUM OF HUMAN HEMOGLOBIN IN CONCENTRATED SODIUM CHLORIDE SOLUTIONS . CONSISTENCY WITH (ALPHABETA) DIMER MODEL

This paper reports a study of the oxidation-reduction equilibrium of human hemoglobin in 2 and 5 m sodium chloride, as a function of pH. The shape of the oxidation- reduction equilibrium curve at high ionic strength is pH dependent, the value of n increasing from 1.4 at pH 6.5 to 2 at pH 8. Over approximately the same pH range the value of n for the redox equilibrium at low ionic strength (0.2) changes from about 1.2 to about 2.5 (Antonini, E., Wyman, J., Brunori, M., Taylor, J. F., Rossi-Fanelli, A., and Caputo, A. (1964), J. Biol. Chem. 239, 907). Therefore at alkaline pH (8-9) there is a decrease in the apparent value of n produced by increasing the ionic strength from 0.2 to 2.2. These results are consistent with the model which proposes that the (αβ) dimer is the functionally important unit in the redox equilibrium of human hemoglobin. The oxidation Bohr effect as a whole is reduced in 2 and 5 m sodium chloride, as compared with that at low ionic strength. However, the “residual” oxidation Bohr effect is still present and unmodified. The constancy of the residual oxidation Bohr effect under different conditions suggests that we are dealing with a direct effect, such as would result from a local conformational change in the neighborhood of the heme. A comparative analysis of the primary structure of myoglobin and hemoglobin, and of the three-dimensional structure of myoglobin, permits a tentative though admittedly speculative assignment of the amino acid side chains involved in the residual oxidation Bohr effect. © 1969, American Chemical Society. All rights reserved.